Weed competition in southern Brazil is one of the main limiting factors for Oryza sativa L. (flooded rice) yield. Echinochloa spp. (barnyardgrass) occurs at a high frequency. Although the potential for weed interference in this cereal is well known, there is little information available about the impact of nutrient competition on rice. Thus, this study aimed to evaluate the relationship between the increase of the barnyardgrass population and the development and nutrition of flooded rice plants at different stages of development. The treatments consisted of growing populations of barnyardgrass competing with the crop from stage V4, which were: 0, 1, 6, 13, 100 and 200 plants · m−2. The experimental design was randomized blocks with two replications, and the experimental units were plots 1.53 m wide by 5 m long. Plant biomass, nutrient uptake and loss of productivity were determined with three replications. An increase in the barnyardgrass population reduced the dry mass of rice leaves and stems, regardless of the evaluation period and the vegetative or reproductive period. Barnyardgrass plants had a significant impact on the reduced grain yield of a flooded rice crop, mainly due to high nutrient competitiveness, especially N in the vegetative period and K in the reproductive period. Barnyardgrass caused a loss of yield by unit · m−2 of 1.13%. The competition for N between rice and barnyardgrass plants was higher in the vegetative period, while for K, Ca and Mg the highest competition occurred in the reproductive period.
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